Transparent conductors are widely used in the flat-panel display industry to form electrodes for electrically switching light-emitting or light-transmitting properties of a display pixel, for example in liquid crystal or organic light-emitting diode displays. Transparent conductive electrodes are also used in touch screens in conjunction with displays. In such applications, the transparency and conductivity of the transparent electrodes are important attributes. In general, it is desired that transparent conductors have a high transparency (for example, greater than 90% in the visible spectrum) and a low electrical resistivity (for example, less than 10 ohms/square). Auxiliary electrodes or wires are used in conjunction with transparent conductive oxides to provide additional conductivity, particularly in areas of a substrate that are not visible to a user.
Conventional transparent conductors are typically coated on a substrate to form a patterned layer of a transparent, conductive material, such as indium tin oxide or other metal oxides, using for example sputtering and photo-lithographic processes. Such materials are increasingly expensive and relatively costly to deposit and pattern. Moreover, metal oxides have a limited conductivity and transparency, and tend to crack when formed on flexible substrates.
Referring to FIG. 4, a prior-art conductive structure includes a substrate 40 with a substrate surface 41. A patterned layer of a transparent conductive oxide 24, such as indium tin oxide, is formed on substrate surface 41, for example by sputtering material onto substrate surface 41 and then photo-lithographically patterning the sputtered material. Photo-lithographic techniques such as blanket coating with a UV-sensitive resin, pattern-wise exposing the resin, washing the unexposed resin to expose the sputtered material layer, etching the exposed sputtered material layer, and stripping away the exposed resin are well known in the integrated circuit manufacturing arts.
A patterned wire 50 is formed on the substrate surface 41 in the substrate areas without conductive material. If the substrate 40 is formed from a non-conductive material, the patterned wire 50 is electrically isolated from the transparent conductive oxide 24. However, sputtering materials is slow and expensive and the photo-lithographic patterning includes many steps, increasing the cost of such patterned conductive structures. Furthermore, transparent conductive oxides tend to crack when exposed to mechanical stress and are therefore problematic when combined with a flexible substrate 40.
Referring to FIG. 5, another prior-art conductive structure includes a substrate 40 with a substrate surface 41. A patterned layer of a conductive polymer 20, such as polyethylene dioxythiophene (PEDOT), is formed on substrate surface 41, for example by coating onto substrate surface 41 and then pattern-wise exposed to a deactivating agent to form conductive areas 21 and less-conductive deactivated areas 22. A patterned wire 50 is formed on a deactivated area 22, for example using photo-lithographic methods or patterned deposition methods such as screen printing or inkjet deposition. However, deactivated area 22 of conductive polymer layer 20 retains some conductivity so that patterned wire 50 is not completely electrically isolated from conductive areas 21. Thus, leakage current 60 can flow between electrically active elements (e.g. wires 50 or conductive areas 21).
There is a need, therefore, for an improved transparent conductive structure.